In the field of transmitting data it has been developed a number of techniques to increase the data rate during transmissions. One technique is to use multiple modulated data streams. A technique called Multiple Input Multiple Output (MIMO) has been defined in WCDMA, Wideband Code Division Multiple Access, Rel-7 to enable higher data rates in downlink as well as uplink. As an example, the peak rate of HSPA, High Speed Packet Access, has been doubled with MIMO compared to earlier releases. In HSPA, MIMO is supported, as an optional feature, by certain categories of User Equipment, UE. User equipment may be any wireless communications device such as mobile phone, PDA, smart phone, laptop, or the like. A MIMO function enables usage of multiple transmit antennas and multiple receive antennas increasing the rate of transmission. For MIMO capable terminals, the MIMO function can be turned on/off by the control element of the network, such as an RNC, Radio Network Controller, or the like; that is, the user equipment is set in a so-called MIMO mode by the RNC.
To support DL MIMO, control channels in uplink, UL, and downlink, DL, have been modified accordingly. The DL HS-SCCH, High Speed—Shared Control Channel, has been extended to incorporate the necessary information for transmitting up to two transport blocks simultaneously. Similarly, the UL control channel, HS-DPCCH, High Speed—Dedicated Physical Control Channel has been extended to incorporate CQIs, Channel Quality Indicator, for up to two “streams” in addition to precoding information. Hence, the payload in the control channels is increased, and due to the increased payload in the MIMO control channels, the coverage for a terminal in MIMO mode is somewhat smaller than for a corresponding terminal not in MIMO mode. Alternatively, more power is needed for control channels to retain the same coverage when a terminal is entering MIMO mode.
Each MIMO capable user equipment may be put into MIMO mode by higher layer signaling. Thus, the RNC can send a MIMO on/off message to the user equipment and base station. This command may be sent at any time, but is mostly foreseen at call setup, cell changes or the like. The base station may then start using the MIMO HS-SCCH and then transmit on the HS-PDSCH, High Speed—Physical Downlink Shared Channel, in MIMO mode. As the RNC transmits the MIMO command an activation time is set based on an estimated time on how long it will take the MIMO command to reach both the base station and the user equipment. The RNC has no continuous feedback from the base station or the user equipment and is not responding to multiple modulated data streams capacity changes within the network.